1. Field of the Invention
The invention relates to a microlithography projection system, a projection exposure system and a chip manufacturing method.
2. Description of Related Art
Lithography employing wavelengths ≦248 nm, preferably ≦193 nm, in particular EUV lithography employing λ=11 nm and/or λ=13 nm, is discussed as a possible technique for imaging structures <130 nm, or more highly preferable <100 nm. The resolution of a lithographic system is described by the following equation:
      RES    =                  k        1            ·              λ        NA              ,where k1 designates a specific parameter of the lithographic process, λ the wavelength of the incident light and NA the image-side numerical aperture of the system.
The optical components of EUV imaging systems are essentially reflective systems employing multilayer coatings. Mo/Be systems are the preferred multilayer coating systems employed for λ=11 nm and Mo/Si systems for λ=13 nm. Lithography with wavelengths below 11 nm is possible.
In order to achieve the highest resolution possible, it is necessary for the system to have an image-side aperture that is as large as possible.
It is an advantage for a lithographic system if the optical path within a projection system or projection lens is free of vignetting or obscuration. The disadvantage of systems with a vignetted exit pupil, e.g. so-called Schwarzschild mirror systems, is that structures of a particular magnitude can only be imaged with reduced contrast. The exit pupil is defined as the image of the aperture stop, imaged by a microlithography projection system located in the optical path between the aperture stop and the image plane.
For instance, 4-mirror systems for microlithography are known from US 2003/0147130, US 2003/0147149, U.S. Pat. Nos. 6,213,610, 6,600,552 or 6,302,548. Systems of this kind are provided with a diaphragm arranged in the optical path from an object plane to an image plane in front of the first mirror of the projection system, a so-called ‘front stop’. A front stop has the disadvantage that it results in either a system with a large overall construction length or a large chief ray angle at the object. A large construction length prevents the construction of a compact and space-saving system and a large chief ray angle at the object generates significant vignette effects when using a reflective mask because the thickness of the absorbent structure mounted on the reflective multiple layers is not negligible.
6-mirror systems for microlithography are known from the publications U.S. Pat. Nos. 6,353,470, 6,255,661, US 2003/0147131 and US 2004/0125353.
In U.S. Pat. No. 6,353,470 the diaphragm lies either on a mirror or between two mirrors, where the distance to a beam of light extending in the vicinity of the diaphragm from one used area of a mirror to a used area of a subsequent mirror in the optical path is <5% of the construction length of the projection system. The construction length or the structural length of a projection system is defined in this application as the axial distance measured along the optical axis (HA) of the projection system from the object plane to the image plane. For a conventional construction length of 1000 mm to 1500 mm for such a system, the radial distance between the aperture stop and a passing beam of light amounts to less than 50 mm or 75 mm, respectively. Within the scope of this application, the radial distance of a used area or an aperture stop from a beam of light is defined as the perpendicular distance with respect to the optical axis from a beam of light which is closest to the boundary of the used area or the aperture stop as shown in FIG. 4b. U.S. Pat. No. 6,255,661 discloses the provision of an aperture stop between the second and third mirror. However, in this case the radial distance to a beam of light extending in the vicinity of the aperture stop from one used area of a mirror to a second used area of a subsequent mirror positioned in the optical path amounts to less than 5% of the construction length of the projection system.
In the case of the 6-mirror system disclosed in US 2003/0147131, the aperture stop lies on the second mirror or between the first and the second mirror. With an arrangement between the first and the second mirror, the radial distance of the aperture stop to the optical path from the object to the first mirror and to the optical path from the second to the third mirror is less than 5% of the construction length of the projection lens.
U.S. Pat. No. 6,781,671 discloses a 6-mirror system that has a aperture stop arranged between the second and third mirror. The radial distance of the aperture stop to the optical path from the first to the second mirror is less than 11% and the radial distance of the aperture stop to the optical path from the third to the fourth mirror less than 32% of the construction length. The chief ray angle at the diaphragm is greater than 26°. In the present application the chief ray angle at the diaphragm or aperture stop is defined as the angle at which the chief ray of the central field point passes through the diaphragm plane in which the diaphragm or aperture stop is arranged.
U.S. Pat. No. 6,781,671 discloses an 8-mirror system that has a aperture stop arranged between the second and third mirror. The radial distance of the aperture stop to the optical path that extends from the first to the second mirror, amounts to less than 12%, and the distance of the aperture stop to the optical path from the third to the fourth mirror is less than 16%. The chief ray angle at the aperture stop is greater than 24°.
Further 8-mirror systems for microlithography are known from US 2002/0129328 or U.S. Pat. No. 6,556,648. In these systems the aperture stops are always positioned on a mirror.
U.S. Pat. No. 5,686,728 shows an 8-mirror system having an aperture stop between the second mirror and the third mirror, but the radial distance of the aperture stop in this system to the optical path of a beam of light extending in the vicinity of the aperture stop, is less than 1% of the construction length of the projection system.
Systems, as described above, in which the aperture stop lies on or near a mirror, have the disadvantage that an adjustable aperture stop design can only be technically implemented at a certain minimum distance of the diaphragm in front of a mirror. Consequently, the light passes through an aperture stop arranged in the optical path in this manner twice: once directly in front of the mirror and once directly after the mirror. This results in vignetting by the diaphragm that becomes apparent for instance in H-V differences. In order to prevent this vignetting, a single-pass diaphragm or aperture stop is advantageous from an optical standpoint, in particular with high aperture systems.
A single-pass aperture stop for an 8-mirror system is shown in U.S. Pat. No. 5,686,728. The aperture stop in U.S. Pat. No. 5,686,728 is arranged between the second mirror and the third mirror in the front section of the system. The disadvantage of this embodiment is, however, the large chief ray angle at the diaphragm, which is approximately 34° for an image-side NA of 0.5. The large chief ray angle at the aperture stop in U.S. Pat. No. 5,686,728 is attributable to the minimal axial distance between the second mirror and the third mirror. In order to ensure an obscuration-free optical path in the case of U.S. Pat. No. 5,686,728, it is necessary to physically split the beam of light at the aperture stop from the first mirror to the second mirror, from the second mirror to the third mirror and from the third mirror to the fourth mirror. In order to achieve this, the projection system has a large chief ray angle at the aperture stop as this results in a smaller aperture diameter, owing to the fact that the product of the chief ray angle and the diameter of the beam of light is a constant. However, a large chief ray angle at the aperture stop has disadvantages. For instance, a large chief ray angle causes a large telecentric error when the diaphragm is displaced along an x, y or z axis. A further disadvantage is that a large chief ray angle means the diameter of the aperture stop is small. This has technical manufacturing disadvantages as requirements with respect to the precision of shaping measured in absolute units are very stringent for aperture stops or diaphragms with a small diameter. In contrast, a small chief ray angle means a large aperture diameter and less stringent requirements with respect to precision of shaping or mould precision.